We have studied the surface composition of gallium arsenide and indium phosphide under conditions of physical sputtering and reactive ion beam etching. Samples of Fe‐doped (100) GaAs and InP were bombarded with 1‐keV Ne+ions under a varying amount of Cl2dose. Low‐energy ion scattering spectroscopy and sputtered neutral mass spectrometry indicate an increase of the Group V element at the surface upon addition of Cl2to the system. This effect is believed to be due to segregation of the As and P due to an altered chemical potential at the surface/vacuum interface resulting from chlorine adsorption. The segregation and subsequent volatilization of PClxspecies leaves aggregates of In/InClxspecies at the surface, resulting in a roughened surface.